Method of producing plug for piercing-rolling

ABSTRACT

A method for producing a plug for use in a piercing rolling mill for producing a seamless steel tube/pipe includes an arc-spraying step of melting iron wires, and spraying molten material thereof onto a surface of a base metal of a plug by use of an arc-spray gun, so as to form a film containing oxide and Fe on the surface of the base metal of the plug. In the arc-spraying step, the surface of the base metal of the plug is divided into plural sections along an axial direction of the plug, and in turn, the arc-spraying is separately carried out in each of the plural sections while an intersection angle between the center line of a spraying stream from the arc-spray gun and the surface of the plug base metal is maintained within a range of 35 degrees to 90 degrees.

TECHNICAL FIELD

The present invention relates to a method for producing a plug for piercing-rolling for use in a piercing-rolling mill (hereinafter, also referred to simply as a “piercer”) that produces a seamless steel tube/pipe, particularly to a method for producing a plug for piercing-rolling having a film formed by performing arc-spraying of iron wires on a surface of a plug base metal.

BACKGROUND ART

A seamless steel tube/pipe is produced by the Mannesmann tube-making process. The Mannesmann tube-making process includes the following steps:

(1) piercing-rolling a starting material (round billet) heated at a predetermined temperature into a hollow shell by using a piercer;

(2) elongation-rolling the hollow shell by an elongation rolling mill (e.g. mandrel mill); and

(3) carrying out diameter adjusting rolling on the elongation-rolled hollow shell to have a predetermined outer diameter and wall thickness by using a diameter adjusting rolling mill (e.g. a stretch reducer).

In the piercing-rolling by using the piercer, a plug is used as a piercing tool. This plug is mounted to a front end of a mandrel so as to pierce a billet heated at a high temperature of approximately 1200° C.; thus the plug is subjected to a hostile environment with a high surficial pressure and a high temperature. In general, the plug includes a base metal made of hot working tool steel, and a film of oxide scale is formed on a surface of the base metal through a heating process in advance for the purpose of protection of the base metal, and thereafter the plug is used in the piercing-rolling. During the piercing-rolling, the scale film on the surface of the plug insulates heat transfer from the billet to the base metal of the plug, and also prevents seizing between the billet and the plug.

Repetitive piercing-rolling using such a plug having the scale film causes a gradual abrasion of the scale film. The abrasion of the scale film deteriorates thermal insulation effect of the film, resulting in increase in temperature of the plug during the piercing, so that melting-incurred metal loss and deformation by heat are likely to be caused to the plug base metal. If the scale film is exhausted, and the plug base metal comes into direct contact with the billet, seizing is caused, so as to generate flaws on an internal surface of a steel tube/pipe. Consequently, the plug becomes unusable at the moment when the film is exhausted, and its durability life is expired.

Particularly in production of a seamless steel tube/pipe made of high alloy steel such as high Cr steel containing Cr of 9% or more, Ni-based alloy, and stainless steel, significant abrasion of the scale film on the surface of the plug is generated during the piercing-rolling, so that the durability life of the plug becomes significantly reduced. For example, in the case of piercing stainless steel, the scale film on the surface of the plug becomes worn away through two or three passes (the number of times of continuous piercing rolling), and the durability life of this plug is expired. This requires a frequent replacement of the plug, which deteriorates the production efficiency. In production of a seamless steel tube/pipe of high alloy steel, it is required to enhance the durability life of the plug during the piercing-rolling, thereby enhancing the production efficiency of the steel tube/pipe.

To satisfy such a requirement, as an example of the film formed on the surface of the plug base metal, Patent Literature 1 discloses such a plug having a film containing oxide and Fe formed on the surface of the plug base metal by performing arc-spraying of iron wires, instead of using the scale film formed through heat treatment. Since the plug having the arc-sprayed film has a film containing oxide and Fe on the surface of the plug, this plug is excellent in thermal insulation performance and seizing prevention, so that enhancement of the durability life of the plug is likely to be achieved.

Patent Literature 1 discloses an equipment system of producing (reproducing) a plug having an arc-sprayed film by forming the film containing oxide and Fe on a surface of a base metal of the plug in such a manner that, after shotblasting is applied onto the surface of the plug, molten material is sprayed from arc-spray guns onto the surface of the plug base metal while a turntable on which the plug is mounted is being rotated. In this equipment system, the spray guns are so disposed as to face a tip end portion, a front-half of the body portion, and a rear-half of the body portion of the surface of the plug base metal, and forms the arc-sprayed film by operating all the spray guns at the same time, thereby reducing time required for forming the film compared to the case of using a single spray gun to form the arc-sprayed film across the entire surface of the plug base metal, which results in enhancement of production efficiency of the plug.

Unfortunately, even in the plug having the arc-sprayed film formed by using the conventional equipment system disclosed in Patent Literature 1, there occurs separation of the film if a billet length to be pierced is long, or if a billet having high elevated-temperature strength is used. This is due to the fact that the adhesiveness of the film is unstable. In this regard, there is still room for further improvement in securing the steadily enhanced durability life of the plug.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent No. 4279350

SUMMARY OF INVENTION Technical Problem

An object of the present invention, which has been made in order to solve the problems according to the conventional art, is to provide a method for producing a plug for piercing-rolling having a film containing oxide and Fe formed on a surface of the plug base metal by performing arc-spraying of iron wires, and the method has the following features of:

(1) securing firm adhesiveness of the film formed on the surface of the plug; and

(2) securing steady enhancement of the durability life of the plug even if a billet length to be pierced is long, or even if a billet having high elevated-temperature strength is used.

Solution to Problem

The summary of the present invention is as follows.

Provided is a method for producing a plug for use in a piercing-rolling mill for producing a seamless steel tube/pipe, and

the method for producing the plug for piercing-rolling comprises

an arc-spraying step of melting iron wires, and spraying molten material thereof onto a surface of a base metal of a plug by use of an arc-spray gun, so as to form a film containing oxide and Fe on the surface of the base metal of the plug.

In the arc-spraying step, the surface of the base metal of the plug is divided into plural sections along an axial direction of the plug, and in turn, the arc-spraying is separately carried out in each of the plural sections while an intersection angle between the center line of a spraying stream from the arc-spray gun and the surface of the plug base metal is maintained within a range of 35 degrees to 90 degrees.

In this method of producing the plug, it is preferable that the plug has a bullet shape, and includes a body portion and a tip end portion, while the plural sections comprise a region of the body portion and the tip end portion.

Advantageous Effects of Invention

The method for producing a plug for piercing-rolling according to the present invention achieves the following remarkable effects of:

(1) securing firm adhesiveness of the arc-sprayed film formed on the surface of the plug; and

(2) securing steady enhancement of the durability life of the plug even if a billet length to be pierced is long, or even if a billet having high elevated-temperature strength is used.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration showing the state of the arc-spraying conducted in basic tests for investigating adhesiveness of the arc-sprayed film.

FIG. 2 is an illustration showing dependency on the intersection angle between the center line of a spraying stream of an arc-spray gun and the surface of the base metal of the plug as a result of the basic tests on the adhesiveness of the arc-sprayed film.

FIG. 3 is an illustration showing microscopic observation photographs of the cross sections of each film depending on the intersection angle between the center line of the spraying stream of the arc-spray gun and the surface of the base metal of the plug as the result of the basic tests on the adhesiveness of the arc-sprayed film.

FIG. 4 is a schematic illustration explaining a conventional film forming method through the arc-spraying, and an operation condition of the arc-spraying of the Comparative Example 4.

FIG. 5 is a schematic illustration explaining the reason why enhancement of the durability life of the plug cannot be achieved by forming the arc-sprayed film in the conventional method shown in FIG. 4.

FIG. 6 is a schematic illustration showing the steps of the film formation through the arc-spraying in the production method of the plug according to the first embodiment of the present invention.

FIG. 7 is a schematic illustration showing the steps of the film formation through the arc-spraying in the production method of the plug according to the second embodiment of the present invention.

FIG. 8 is a schematic illustration explaining an operation condition of the arc-spraying of the Comparative Example 1.

FIG. 9 is a schematic illustration explaining an operation condition of the arc-spraying of the Comparative Example 2.

FIG. 10 is a schematic illustration explaining an operation condition of the arc-spraying of the Comparative Example 3.

FIG. 11 is a schematic illustration explaining an operation condition of the arc-spraying of the Inventive Example 1.

FIG. 12 is a schematic illustration explaining an operation condition of the arc-spraying of the Inventive Example 2.

FIG. 13 is a schematic illustration explaining an operation condition of the arc-spraying of the Inventive Example 3.

DESCRIPTION OF EMBODIMENTS

In order to achieve the above object, the present inventors conducted various tests and intensive studies on forming a film containing Fe oxide and Fe on a surface of a plug base metal by performing arc-spraying of iron wires onto the surface of the plug base metal, focusing attention on the adhesiveness of the arc-sprayed film in particular. As a result, the present inventors have obtained the following findings.

The arc-spraying is a technique, for example, to use an arc-spray gun, and generates arcs between front ends of two spray wires serving as electrodes, so as to melt the spray wires, and at the same time, supply a compressed air jet or a nitrogen gas jet between the front ends of the spray wires so that the molten material is sprayed, thereby spraying the molten material onto a target object to form a film thereon. In the arc-spraying in which iron wires are applied as the spraying wires, and a plug is used as the target object, a film formed on the surface of the plug base metal contains Fe oxide (iron oxide) and Fe. The Fe oxide contained in the film results from molten material (molten iron) that is sprayed from the arc-spray gun, and is oxidized in the air before the molten material reaches the surface of the plug base metal. Fe contained in the film results from the molten material that has reached the surface of the plug base metal before being oxidized in the air.

FIG. 1 is a schematic illustration showing the state of the arc-spraying conducted in the basic tests for investigating the adhesiveness of the arc-sprayed film. As shown in this illustration, in the basic tests for investigating the adhesiveness of the film, molten material resulted from the iron wires is sprayed from the arc-spray gun 4 while the plug base metal 2 is being rotated around the central axis Pc of the plug 1, so as to form the film on the surface of the plug 1. At this time, various films were formed by varying an intersection angle θ defined by the center line Ac of a spraying stream from the arc-spray gun 4 and the surface of the base metal 2 of the plug 1. As an evaluation procedure of the adhesiveness of the film, a peel stress in the shear direction of the film (hereinafter referred to as “adhesiveness”) was measured for each of the plugs 1 having different intersection angles, referred to as θ. The film adhesiveness for the plug in case of the intersection angle θ of 90 degrees was defined as a reference “1”, and the evaluation of film adhesiveness was conducted based on the ratio of the film adhesiveness (adhesiveness ratio) of each plug having a different intersection angle θ relative to this reference. The microscopic observation of the cross section of the film of each plug was also conducted.

FIG. 2 is an illustration showing the dependency on the intersection angle between the center line of the spraying stream of the arc-spray gun and the surface of the base metal of the plug as a result of the basic tests on the adhesiveness of the arc-sprayed film. FIG. 3 is, as the result of the basic tests, an illustration showing microscopic observation photographs of the cross sections of plugs in case of different intersection angles, each defined by the center line of the spraying stream of the arc-spray gun and the surface of the base metal of the plug.

As shown in FIG. 2, the adhesiveness ratio of the film depends on the intersection angle θ between the center line of the spraying stream of the arc-spray gun and the surface of the base metal of the plug. Specifically, if the intersection angle θ is smaller than 35 degrees, the adhesiveness ratio is apt to significantly decrease. To the contrary, if the intersection angle θ is 60 degree or more, there is no sign of the decrease in the adhesiveness ratio.

As shown in FIG. 3, the reason for the deterioration of the adhesiveness in case of a smaller intersection angle θ is because the film might ununiformly adhere onto the surface of the plug base metal, which results in increase of the percentage of porosity in the film.

Here, the arc-spraying is generally used in the repair of a tapping port of a metal refining vessel formed of a refractory material, or in coating on an internal surface of a cylinder bore of an engine. In this case, the target of the arc-spraying is the internal surface of a cylindrical member, and is carried out such that a spray gun is inserted in a cylindrical member that is immobilized, so that the distance between the spray gun to the target surface on which the film is to be formed, that is, the spraying distance is approximately 50 mm, or approximately 150 mm at most, which is small. In such a general arc-spraying, it is not preferable to set the intersection angle between the center line of the spraying stream of the spray gun and the target surface for the film formation to be a large angle. If the intersection angle is large, molten material sprayed from the spray gun splashes back from the target surface for the film formation, and is then returned to the spray gun; therefore damages are caused to the spray gun, or the molten material splashed back from the target surface for the film formation is inadvertently re-sprayed onto the target surface for the film formation, which deteriorates the adhesiveness of the film; thus a larger intersection angle is not preferable in light of prevention of the above undesirable incidents.

According to this theory, there might be a risk that, in the arc-spraying for the plug as a target surface of the film formation, a larger intersection angle between the center line of spraying stream of the spray gun and the surface of the plug base metal may also reduce the adhesiveness of the film. As described above, in the arc-spraying for the plug, however, a larger intersection angle θ rather secures enhancement of the adhesiveness of the film. The reason for this is as follows.

In a case of the arc-spraying with iron wires to form the film containing oxide and Fe on the surface of the plug base metal, it is required to secure sufficient time for oxidizing the molten material sprayed from the spray gun in the air; thus the spraying distance from the spray gun to the surface of the plug base metal, i.e., standoff distance from the surface is approximately 200 to 1000 mm, which is relatively large. Accordingly, even if the intersection angle is set to be large, the splash back of the molten material hardly occurs from the surface of the plug base metal.

In the formation of the arc-sprayed film on the surface of the plug base metal, the arc-spraying is carried out while the plug is being rotated, the molten material that should have splashed back from the surface of the plug base metal is tremendously flicked off by the rotation of the plug, so that the molten material is prevented from inadvertently adhering to the surface of the plug base metal.

Based on the above basic tests, it is recognized that, in order to secure the adhesiveness of the film formed on the surface of the plug as well as allow this adhesiveness to have sufficient strength, it is preferable to maintain the intersection angle θ between the center line of spraying stream from the arc-spray gun and the surface of the base metal of the plug within the range of 35 degrees to 90 degrees while the arc-spraying is being carried out to form the arc-sprayed film on the surface of the plug base metal. It is more preferable to set the intersection angle θ within the range of 60 degrees to 90 degrees.

An example of a method for carrying out the arc-spraying with the intersection angle θ within the preferable range may include the following method.

FIG. 4 is a schematic illustration explaining a conventional film forming method through the arc-spraying. As shown in this illustration, in the conventional method, the plug 1 has a bullet shape, and the film 3 is formed on the surface of the plug base metal 2 through the arc-spraying in such a manner that the spray gun 4 is reciprocatingly moved from the rear end to the tip end along the surface of the plug base metal 2 while the plug base metal 2 is being rotated about the central axis P_(c) of the plug 1. In this manner, the film 3 is formed from the tip end portion 1 a, the front-half 1 ba of the body portion 1 b, to the rear-half 1 bb of the body portion 1 b (reeling portion) across the entire surface of the plug 1. At this time, the spray gun 4 is mounted to an articulated arm that is operated by programming, and the motion and posture of the spray gun 4 is controlled by the programming

In a case of forming the film in the conventional method shown in FIG. 4, as verified in Example section described later, the durability life of the plug cannot be enhanced as much as expected. The reason for this is as follows.

FIG. 5 is a schematic illustration showing the reason why enhancement of the durability life of the plug cannot be achieved when the arc-sprayed film is formed by the conventional method shown in FIG. 4. In the conventional method shown in FIG. 4, the spray gun 4 is configured to move in a wide range from the rear end to the tip end of the surface of the plug base metal 2, and thus it is extremely complicated to control the motion and the posture of the spray gun 4. Consequently, as shown in FIG. 5, if a slight deviation occurs in the position adjustment or the posture adjustment of the spray gun 4 relative to the plug base metal 2, the intersection angle θ between the center line Ac of spraying stream from the arc-spray gun 4 and the surface of the plug base metal 2 may be deviated out of the above preferable range (encircled portion in FIG. 5). Due to this, the adhesiveness of the film becomes partially reduced.

Contrary to the conventional method, as verified in the Example section described later, it is found that the durability life of the plug can be significantly enhanced by employing a method that divides the surface of the plug base metal into plural sections along an axial direction of the plug, and in turn, separately forms part of the arc-sprayed film in each section.

Patent Literature 1 describes a technique of forming the arc-sprayed film such that disposes spray guns are disposed opposite to the tip end portion, the front-half of the body portion, and the rear-half of the body portion of the plug, respectively, and operates all the spray guns to form the arc-sprayed film at the same time in order to reduce time required for the film formation through the arc-spraying. Even in this technique, the durability life of the plug cannot also be enhanced as much as expected, similarly to the conventional method shown in FIG. 4, and the reason for this is not identified yet.

The present invention has been made based on the above findings. Hereinafter, provided is the description on the preferable embodiments of the method for producing the plug of the present invention.

First Embodiment

FIG. 6 is a schematic illustration showing the steps of the film formation through the arc-spraying in the production method of the plug according to the first embodiment of the present invention, FIG. 6( a) shows the state of forming the film at the body portion of the plug as step 1, and FIG. 6( b) shows the state of forming the film at the tip end portion of the plug as step 2. The method of the first embodiment shown in this illustration is based on the configuration of the conventional method shown in FIG. 4, and a duplicate description will be omitted when appropriate.

In the method of the first embodiment, as shown in FIG. 6, the plug 1 has a bullet shape, and the film 3 is formed on the surface of the plug base metal 2 through the arc-spraying while the plug base metal 2 set on a turntable (not shown) or the like is being rotated about the central axis P_(c) of the plug 1. Specifically, the surface of the plug base metal 2 is divided into two sections along an axial direction of the plug 1. FIG. 6 shows an example of the plug 1 divided into the tip end portion 1 a and the body portion 1 b. The body portion 1 b is a portion of 80 to 98% of the overall length from the rear end (lower end in the illustration) of the plug 1 in an axial direction (vertical direction in the illustration) of the plug 1.

Based on the above configuration, in step 1 as shown in FIG. 6( a), the arc-spraying is carried out such that the spray gun 4 is disposed opposite to a region of the body portion 1 b of the plug among the surface of the plug base metal 2, and is reciprocatingly moved along the surface of this region only. In this manner, the film 3 is formed on the body portion 1 b other than the tip end portion 1 a on the surface of the plug base metal 2.

Subsequently to the above step, in step 2 as shown in FIG. 6( b) the arc-spraying is carried out such that the spray gun 4 is disposed opposite to a region of the tip end portion 1 a of the plug among the surface of the plug base metal 2, and is reciprocatingly moved for a short distance along the surface of this region only. In this manner, the film 3 is formed at the tip end portion 1 a on the surface of the plug base metal 2. Accordingly, the film 3 is formed across the entire surface of the plug 1.

At this time, in both of the steps 1, 2 shown in FIG. 6( a) and FIG. 6( b), the motion and the posture of the spray gun 4 are controlled in such a manner that the intersection angle θ between the center line A_(c) of spraying stream from the spray gun 4 and the surface of the plug base metal 2 is within the preferable range that is found based on the result of the above described basic tests, that is, within the range of 35 degrees to 90 degrees, more preferably of 60 degrees to 90 degrees.

According to the method of the present embodiment, it is configured that the surface of the plug base metal 2 is divided into two sections (the tip end portion 1 a and the body portion 1 b) in an axial direction of the plug 1, and in turn, the arc-spraying is carried out separately in each of two sections with the intersection angle θ between the center line A_(c) of spraying stream from the spray gun 4 and the surface of the plug base metal 2 maintained within the preferable range, so as to form the film 3 through the arc-spraying, thereby producing the plug having the arc-sprayed film with firm adhesiveness between the plug base metal and the film as well as significantly enhanced durability life of the plug. In addition, it is possible to reduce the operational range of the spray gun 4 at the time of the arc-spraying in each divided section; thus the above intersection angle θ can be securely maintained within the preferable range without controlling the motion and the posture of the spray gun 4 in a complicated manner. Accordingly, the adhesiveness of the film becomes stable across the entire surface of the plug, thereby realizing stable durability life of the plug, as well.

Second Embodiment

FIG. 7 is a schematic illustration showing the steps of the film formation through the arc-spraying in the production method of the plug according to the second embodiment of the present invention; and FIG. 7( a) shows the state of forming the film at the rear-half of the body portion of the plug as step 1, FIG. 7( b) shows the state of forming the film at the front-half of the body portion of the plug as step 2, and FIG. 7( c) shows the state of forming the film at the tip end of the plug as step 3. The method of the second embodiment is based on the configuration of the above first embodiment, while divided sections of the surface of the plug base metal 2 are further increased. In the second embodiment, the surface of the plug base metal 2 is divided into three sections. FIG. 7 shows an example of the plug 1 divided into the tip end portion 1 a, the front-half 1 ba of the body portion 1 b, and the rear-half 1 bb of the body portion 1 b of the plug 1.

In the second embodiment, in step 1 as shown in FIG. 7( a), the arc-spraying is carried out such that the spray gun 4 is disposed opposite to the region of the rear-half 1 bb of the body portion 1 b of the plug among the surface of the plug base metal 2. In this manner, the film 3 is formed on the rear-half 1 bb of the body portion other than those of the tip end portion 1 a and the front-half 1 ba of the body portion on the surface of the plug base metal 2.

Subsequently, in step 2 as shown in FIG. 7( b), the arc-spraying is carried out such that the spray gun 4 is disposed opposite to the region of the front-half 1 ba of the body portion 1 b of the plug among the surface of the plug base metal 2. In this manner, the film 3 is formed on the front-half 1 ba of the body portion on the surface of the plug base metal 2.

Subsequently, in step 3 as shown in FIG. 7( c), the arc-spraying is carried out such that the spray gun 4 is disposed opposite to the region of the tip end portion 1 a of the plug among the surface of the plug base metal 2. In this manner, the film 3 is formed on the tip end portion 1 a on the surface of the plug base metal 2. Accordingly, the film 3 is formed across the entire surface of the plug 1.

In all steps 1 to 3 shown in FIG. 7( a) to FIG. 7( c), the motion and the posture of the spray gun 4 are controlled to carry out the arc-spraying in such a manner that the intersection angle θ between the center line A_(c) of spraying stream from the spray gun 4 and the surface of the plug base metal 2 is within the preferable range that is found based on the above basic tests.

The method of the second embodiment can also achieve similar effects as those by the method of the first embodiment.

The number of the divided sections of the surface of the plug base metal may be any number more than one, and the number of the divided sections may be determined depending on a taper angle and/or the curvature of the surface of the plug.

The plug provided by the above method of each embodiment can be reproduced after its durability life is expired through repetitive piercing-rolling by re-forming the arc-spayed film on the surface of the plug using the same method. Immediately before the re-forming of the film, a jet air injection or the shotblasting is preferably applied onto the surface of the plug, so as to remove the film remaining on the surface of the plug in as-is condition after the repetitive piercing-rolling.

It is preferable to always apply an air jet injection or the shotblasting onto a target region for the arc-spraying of each section immediately before the arc-spraying is applied to this section of the divided surface of the base metal. If molten material of the arc-spraying that is carried out to a different section inadvertently adheres onto the target region for the arc-spraying, it may happen at an inappropriate intersection angle, which may cause ununiformity in the adhesiveness of the film

A shield plate may be disposed so as to cover a region other than a target region for the film formation when the arc-spraying is applied to each section of the divided surface of the base metal. Specifically, in the above first embodiment, the shield plate is so disposed as to cover the tip end portion at the time of applying the arc-spraying to the body portion, and the shield plate is so disposed as to cover the body portion at the time of applying the arc-spraying to the tip end portion. In the second embodiment, similarly to the first embodiment, the shield plate is so disposed as to cover the front-half of the body portion and the tip end portion at the time of applying the arc-spraying to the rear-half of the body portion. The shield plate is so disposed as to cover the rear-half of the body portion and the tip end portion at the time of applying the arc-spraying to the front-half of the body portion, and the shield plate is so disposed as to cover the front-half of the body portion and the rear-half of the body portion at the time of applying the arc-spraying to the tip end portion of the plug. This is to prevent adhesion of the molten material sprayed from the sprayer onto an unexpected region at an unfavorable intersection angle, thereby preventing deterioration of the adhesiveness between the plug base metal and the film. Hence, the shield plate may be so disposed as to cover at least a region where no sprayed film is formed yet, and unnecessary to be disposed at a region where the sprayed film is already formed.

In the method of each embodiment, the film formed through the arc-spraying may have a uniform thickness across the entire surface of the plug, or may have a heavier thickness at the tip end portion than at the body portion of the plug. The film having a heavier thickness at the tip end portion of the plug is useful in light of securing enhanced thermal insulation performance and wear resistance of the film at the tip end portion of the plug where the surficial pressure becomes high and the temperature is increased during the piercing-rolling, so that further enhancement of the durability life of the plug can be expected.

Example

For the purpose of verifying the effects of the present invention, a piercing-rolling test was conducted in such a manner that plugs for piercing-rolling were produced, and each of the produced plugs was mounted to a piercer so as to carry out the piercing-rolling. The test condition was as follows.

[Test Method]

(1) Production of Plug

A number of bullet-shaped plugs, each having a maximum diameter of 57 mm, were produced using hot-working tool steel specified by the JIS standard as the base metal. Plugs having the arc-sprayed film were produced in such a manner that the arc-spraying was carried out by using iron wires under various operation conditions of the arc-spraying, so as to form a film on the surface of the base metal of each plug. In the formation of the arc-sprayed film, the arc-spraying was conducted for each plug with the spraying distance from the spray gun to the surface of the plug base metal initially fixed at 200 mm, and the arc-spraying was carried out while the spray gun was gradually distanced away from the surface of the plug base metal until the spraying distance finally became 1000 mm. The thickness of the arc-sprayed film was set to 400 μm at the body portion of the plug, and 1200 μm at the tip end of the plug.

As a reference for the evaluation, plugs having scale films were produced by forming an oxide scale film on the surface of the base metal of each plug through a heat treat furnace. The thickness of the scale film was set to 600 μm.

Each operation condition of the arc-spraying was as follows.

Comparative Example 1

As shown in FIG. 8, with the center line A, of the spraying stream from the spray gun 4 always maintained to make a right angle with the central axis P_(c) of the plug metal base 2, the arc-spraying is carried out while the spray gun 4 is being moved across the entire region from the rear end to the tip end of the surface of the plug base metal 2. In this example, the intersection angle θ between the center line A, of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is deviated from the preferable range at the tip end portion 1 a of the plug.

Comparative Example 2

As shown in FIG. 9, with the center line A, of the spraying stream from the spray gun 4 always maintained to be parallel to the central axis P_(c) of the plug metal base 2, the arc-spraying is carried out while the spray gun 4 is being moved across the entire regions of the body portion 1 b and of the tip end portion 1 a of the plug. In this example, the intersection angle θ between the center line A, of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is deviated from the preferable range at the body portion 1 b of the plug.

Comparative Example 3

In step 1 as shown in FIG. 10( a), with the center line A, of the spraying stream from the spray gun 4 always maintained to be parallel to the central axis P_(c) of the plug metal base 2, the arc-spraying is carried out to the body portion 1 b of the plug while the sprayer 4 is being moved along only the region of the body portion 1 b of the plug. Thereafter, in step 2 as shown in FIG. 10( b), with the intersection angle θ between the center line A, of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 always being set to 25 degrees, the arc-spraying is carried out to the tip end portion 1 a of the plug while the spray gun 4 is being moved along only the region of the tip end portion 1 a of the plug. In this example, the intersection angle θ between the center line A, of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is deviated from the preferable range across the entire regions of the tip end portion 1 a and of the body portion 1 b of the plug.

Comparative Example 4

As shown in FIG. 4, the posture of the spray gun 4 is controlled so as to carry out the arc-spraying in such a manner that the center line A, of the spraying stream from the spray gun 4 intersects at right angle with the central axis P_(c) of the plug metal base 2 at the rear end of the plug base metal 2, and the center line k of the spraying stream from the spray gun 4 becomes parallel to the central axis P, of the plug metal base 2 at the tip end of the plug base metal 2 while the spray gun 4 is being moved across the entire region from the rear end to the tip end of the surface of the plug base metal 2. In this example, the intersection angle θ between the center line A_(c) of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is deviated from the preferable range in vicinity of the boundary between the tip end portion 1 a and the body portion 1 b of the plug.

Inventive Example of Present Invention 1

In step 1 as shown in FIG. 11( a), the arc-spraying is carried out to the body portion 1 b of the plug by controlling the posture of the spray gun 4 in such a manner that the spray gun 4 is moved along only the region of the body portion 1 b of the plug while the intersection angle θ between the center line A, of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is set to 90 degrees on the rear end side of the body portion 1 b of the plug, and is set to more than 35 degrees on the tip end side of the body portion 1 b of the plug. In step 2 as shown in FIG. 11( b), the arc-spraying is carried out to the tip end portion 1 a of the plug in such a manner that the spray gun 4 is moved along only the region of the tip end portion 1 a of the plug while the intersection angle θ between the center line A, of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is always maintained at 90 degrees. At this time, the intersection angle θ between the center line A, of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is within the preferable range across the entire regions of the tip end portion 1 a and of the body portion 1 b of the plug.

Inventive Example of Present Invention 2

In step 1 as shown in FIG. 12( a), the arc-spraying is carried out to the body portion 1 b of the plug by controlling the posture of the spray gun 4 in such a manner that the spray gun 4 is moved along only the region of the body portion 1 b of the plug while the intersection angle θ between the center line A_(c) of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is always set to 90 degrees. In step 2 as shown in FIG. 12( b), the arc-spraying is carried out to the tip end portion 1 a of the plug in such a manner that the spray gun 4 is moved along only the region of the tip end portion 1 a of the plug while the intersection angle θ between the center line A_(c) of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is always maintained at 45 degrees. In this example, the intersection angle θ between the center line A_(c) of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is within the preferable range across the entire regions of the tip end portion 1 a and of the body portion 1 b of the plug.

Inventive Example of Present Invention 3

In step 1 as shown in FIG. 13( a), the arc-spraying is carried out to the body portion 1 b of the plug by controlling the posture of the spray gun 4 in such a manner that the spray gun 4 is moved along only the region of the body portion 1 b of the plug while the intersection angle θ between the center line A_(c) of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is always set to 90 degrees. In step 2 as shown in FIG. 13( b), the arc-spraying is carried out to the tip end portion 1 a of the plug in such a manner that the spray gun 4 is moved along only the region of the tip end portion 1 a of the plug while the intersection angle θ between the center line A_(c) of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is always maintained at 90 degrees. In this example, the intersection angle θ between the center line A_(c) of the spraying stream from the spray gun 4 and the surface of the plug base metal 2 is within the preferable range across the entire regions of the tip end portion 1 a and of the body portion 1 b of the plug.

(2) Piercing-Rolling

Using the above various plugs, the following hollow shells were produced by repetitively piercing-rolling the following workpieces (materials) heated at 1200° C.

-   -   Workpiece size: round billet of 70 mm in diameter and 600 mm in         length     -   Workpiece material grade: SUS304     -   Hollow shell: 73 mm in outer diameter, 6.0 mm in wall thickness,         1800 mm in length.

[Evaluating Method]

Inspection was conducted on the appearance of each plug every time the piercing-rolling was completed. For each plug, the number of times of rolling pass until the plug became unusable due to the peel-off of the film, or melting-incurred metal loss, seizing or deformation was generated at the tip end of the plug was investigated, in other words, the number of billets that successfully got through the continuous piercing-rolling (number of times of continuous piercing-rolling) was counted. The durability life of the plug was evaluated based on the ratio of the durability life of each plug relative to a reference “1.0” (hereinafter referred to as “plug durability life ratio”), where the plug has the scale film and its durability life is defined as the reference “1.0”.

[Test Result]

Test result is shown in Table 1.

TABLE 1 Classification Satisfiability on Preferable Plug Plug Surface (Reference Range of Intersection Angle Durability Condition after No Illustration) θ (35° to 90° ) Life Ratio Piercing-rolling 1 Comparative Deviated at Plug Tip End 0.5 Seizing at Tip Example 1 Portion End Portion (FIG. 8) 2 Comparative Deviated at Plug Body 1.0 Film Peel-off at Example 2 Portion Body Portion (FIG. 9) 3 Comparative Deviated across Entire Plug 0.5 Film Peel-off Example 3 from Tip End (FIG. 10) Portion to Body Portion 4 Comparative Deviated in Vicinity of 2.5 Seizing at Tip Example 4 Boundary Between Tip End End Portion (FIG. 4) Portion and Body Portion of Plug 5 Inventive Within Preferable Range 6.0 Wrinkle at Body Example 1 across Entire Plug Portion (FIG. 11) 6 Inventive Within Preferable Range 5.0 Seizing at Tip Example 2 across Entire Plug End Portion (FIG. 12) 7 Inventive Within Preferable Range 7.0 Wrinkle at Body Example 3 across Entire Plug Portion (FIG. 13) Note: The durability life of each plug is represented by using its ratio relative to the reference “1.0”, where the durability life of the plug having the scale film is defined as the reference “1”,

The result of Table 1 reveals the following. In the Comparative Examples 1 to 3 shown in Classification Nos. 1 to 3, since most of the films formed through the arc-spraying were produced under the condition in which the intersection angle θ was out of the preferable range (35 degrees to 90 degrees), Peel-off of the film, melting loss or seizing of the plug was caused at an early stage, and the durability life of the plug became 1.0 or less; thus no enhancement of the durability life of the plug could be confirmed. The Comparative Example 4 shown in Classification No. 4 exhibited some enhancement of the durability life of the plug, but the durability life ratio of the plug was 2.5 at most because the arc-spraying was carried out across the surface of the plug base metal at a time without dividing the surface of the plug base metal.

To the contrary, in Inventive Examples 1 to 3 shown in Classification Nos. 5 to 7, the surface of the plug base metal was divided into plural sections, and in turn, the arc-spraying was separately carried out in each section under the condition of satisfying the preferable range of the intersection angle θ (35 degrees to 90 degrees) specified by the present invention. Accordingly, it was confirmed that the adhesiveness of the film was significantly enhanced, and the durability life ratio of each plug became 5.0 or more, which reveals significant enhancement of the durability life of the plug.

INDUSTRIAL APPLICABILITY

The present invention can be effectively used in production of seamless steel tube/pipe of high alloy steel.

REFERENCE SIGNS LIST

-   1: Plug, 1 a: Tip end portion of plug, 1 b: Body portion of plug, -   1 ba: Front-half of body portion of plug, 1 bb: Rear-half of body     portion of plug, -   2: Plug base metal, 3: Arc-sprayed film, 4: Arc-spray gun -   Pc: Central axis of plug, Ac: Center line of spraying stream of     arc-spray gun, -   θ: Intersection angle 

What is claimed is:
 1. A method for producing a plug for use in a piercing-rolling mill for producing a seamless steel tube/pipe, the method comprising: an arc-spraying step of melting iron wires, and spraying molten material thereof onto a surface of a base metal of a plug by use of an arc-spray gun, so as to form a film containing oxide and Fe on the surface of the base metal of the plug, wherein in the arc-spraying step, the surface of the base metal of the plug is divided into plural sections along an axial direction of the plug, and in turn, the arc-spraying is separately carried out in each of the plural sections while an intersection angle between the center line of a spraying stream from the arc-spray gun and the surface of the plug base metal is maintained within a range of 35 degrees to 90 degrees.
 2. The method for producing a plug for piercing-rolling according to claim 1, wherein the plug has a bullet shape, and includes a body portion and a tip end portion, while the plural sections comprise a region of the body portion and a region of the tip end portion. 